According to lincoln, how does a nation make sure that free governments (democracies), "shall not perish from the earth"?

He is saying that every generation must work to preserve what was fought here and remember the sacrifices made during that day.  They gave their lives not only to protect the Union but also for the liberties of all men so everyone would be equal.  This is the best way to ensure that democracy will always be there.

necessary and proper clause. the congress shall have power to all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this constitution in the government of the united states, or in any department or officer thereof.

"they had a large area of land that they owned."